In making high performance devices such a turbine engines, it is important to use materials which are strong, especially at high temperatures. The strength of any material is defined as its resistance to deformation. On an atomic scale, resistance to deformation in a crystalline material corresponds to its resistance to dislocation motion. Dislocation motion, in turn, is resisted by internal stresses. Therefore, in order to achieve maximum strength, the internal stresses in the material should meet the following requirements: act in unison, be as large as possible, not be easily avoidable, and not be thermally surmountable at significant rates.
Mechanisms or techniques used to strengthen conventional alloys include hardening by solid solutions, precipitation and dispersion of particles and strain hardening. However, none of these mechanisms meet all the requirements listed above and thus these mechanisms do not maximize strength. Importantly, conventional strengthening mechanisms lose their effectiveness at high temperature due to thermally activated processes such as creep. For example, when the nickel based superalloys used in turbine engines are strengthened by conventional schemes, the superalloys lose nearly all of their strength at temperatures between seven to eight tenths of the melting point of the nickel matrix. Consequently, in order to obtain dramatic increases in the strength of metal composite materials a new and revolutionary approach is needed.
A key object of the invention to produce a metal composite material which has increased strength at a temperature which is greater than half the melting temperature of one of its component materials.
The present invention concerns a metal composite material of improved strength as well as a process for making the material. The metal composite material according to the invention has a unique structure. In this material, over 50 volume percent is a hard particulate, and the remainder is metal matrix that is significantly more ductile than the particulate material. The particulate and matrix material should be capable of wetting each other, but are preferably inert to each other. In this matrix, each particle is essentially surrounded by a thin coating of the matrix material. By virtue of this thin coating between the particles, deformation caused, for example, by pulling, is translated into rotational movement of the particles about each other, even at high temperature. The translation of deformation stresses into this rotational movement greatly enhance the resistance of the composite to deformation.
In one embodiment, the material composite of this invention is made by placing particles of one material inside a hollow cylinder of a metal matrix material to form a mixture which is greater than 50% particles by volume. The two materials are chosen so that they capable of wetting each other and are essentially immiscible in the solid state. After capping the cylinder with a piece of the matrix material, the cylinder is wrapped in foil and placed in a vacuum resistant jacket. Then air is evacuated from the jacket to form a vacuum. After being evacuated, the jacket is hot isostatically pressed at a temperature above the melting temperature of the matrix material to form a metal composite material having improved strength.
In another embodiment, the composite may be formed by placing a crucible containing the particulate material within a chamber which is evacuated to remove any trapped air or other gas from the spaces between the particles. Within the chamber and above the particulate material is a block of a metal, for example copper. The metal block above the particulate material is heated to above its melting temperature but below the melting temperature of the particulate material. Gravity causes the molten metal to flow downward and infiltrate the particulate material in the crucible.
In a preferred embodiment the particles are less than a micron in size. Additionally, the hot isostatic pressing process may include a second hot isostatic pressing at a temperature below the melting point of the matrix material to give even more strength to the material.
Other features and advantages of the invention will be set forth in, or apparent from, the following detailed description of the preferred embodiments of the invention.